1. Field of the Invention
This invention relates to navigation using a global positioning system (GPS) radio receiver which receives information from earth-orbiting satellites. The invention is particularly applicable to making an initial estimate of the position of the receiver by using satellite tracking data and range rate measurements specific to each acquired satellite, and determining the intersections of the lines having constant range rate values.
2. Description of the Related Art
Embodiments of the present invention may include the process by which a GPS receiver acquires signals transmitted by a constellation of satellites orbiting the earth, and accordingly makes an estimation of the initial receiver position from which to navigate. At any given location on the earth, however, the receiver can only receive signals from those satellites which are visible at that location. Because the satellite signals generally have a low signal-to-noise ratio, acquisition of particular satellites can be improved if the visible satellites which are transmitting signals can be determined.
Typically, GPS receivers are capable of determining which unobscured satellites are transmitting signals by evaluating the signal outputs relative to the location of the receiver itself. However, the receiver often cannot approximate its own location, and thus cannot make a determination of visible satellites based upon a Known location.
The process of detecting signals from visible satellites requires the detection of signals from at least one satellite. The identity of the particular satellite is then determined according to its transmitted information (associated with a pseudo-random code) to select which of the other satellites transmissions to search for next. This process is continued until transmissions are received from the minimum number of GPS satellites necessary for navigation. Four satellites are generally required for three-dimensional tracking.
However, acquiring the necessary satellite signals from a cold start, i.e., when the receiver location and/or time are unknown, is a relatively time consuming process. Accordingly, there is a substantial commercial advantage in reliably performing this acquisition with significantly increased speed. One approach used to improve satellite signal acquisition is to acquire a first signal, then make an initial estimate of the region of the earth in which the receiver is located by calculating a "pseudo range" from the satellite. The pseudo range describes the measurement of range from the receiver to the satellites using an imprecise clock. The pseudo range, however, is an inexact range value due to a bias of fixed magnitude in each range estimate attributable to the clock bias error and clock drift.
The receiver clock bias can significantly affect the resultant range values since the distances, and thus the time lag, between the receiver and the satellites are extremely large. Consequently, the clock bias error magnifies the error in the measured range values. The clock drift error is generally indicated in terms of oscillator frequency. The rate at which the oscillator vibrates corresponds to the frequency, and is thus related to the satellite velocity. Ideally, the clock drift value is assumed to be zero. However, the clock drift error is substantially affected by and varies with temperature. As a result, as soon as the GPS receiver is activated, clock drift or bias error may be expected. Accordingly, it may be important to correct for errors attributable to the clock bias.
In conventional GPS schemes, the receiver searches for signals from satellites known to be visible from the initially-estimated region of probable location. Upon acquisition of another signal, another calculation of pseudo range is made to define a narrower region of location of the receiver, and so on. As the region of location grows smaller, the probability of finding additional satellite signals should improve. As mentioned above, however, the pseudo range approach has an inherent drawback. The calculated value of pseudo range is affected by the above-described bias in the internal clock of the GPS receiver. If the bias is large, the estimated region of location of the receiver may be unreasonably large, and would be correspondingly uncertain. Such a result would be of diminished value.